1. Field of the Invention
The present invention relates to power supplies which allow reduction of power consumption by electronic apparatuses and particularly relates to discharging circuits which can quickly emit remaining charges in a capacitor for noise reduction in a power supply.
2. Description of the Related Art
With reference to FIG. 17A, a discharging circuit will be described which is in an electronic apparatus having a power supply (hereinafter, also called a converter) which receives input of AC voltage from a commercial AC power supply. In a converter which receives input of AC voltage, an X capacitor 40 (popularly called an across-the-line capacitor) which functions as a capacitance element for external noise reduction (also called noise prevention) is generally connected between lines from a commercial AC power supply. For the security after removal of a plug (also called AC plug) by a user, discharging the X-capacitor 40 may be required within one second to a predetermined voltage or lower after the plug is removed. This is provided under Electrical Appliance and Material Safety Act, International Electrotechnical Commission (IEC) and so on, for example. In order to satisfy the specifications, when the X-capacitor 40 is used, a discharging circuit including a discharge resistance 20 is generally provided in parallel with the X-capacitor 40 as illustrated in FIGS. 17A and 17B.
In a converter in a general electronic apparatus, a rectifying unit 8 in the subsequent stage of a discharging circuit performs full-wave rectification on AC voltage, and a converter unit 9 converts it to a voltage for components of the electronic apparatus and supplies it to them. In order to determine the frequency of AC voltage of a commercial AC power supply or detect a power failure instantly, some electronic apparatuses may have a zero-crossing detecting circuit which detects a zero-crossing point of an AC voltage. Such an apparatus may often use a zero-crossing detecting circuit instead of the discharging circuit.
On the other hand, recent electronic apparatuses have been required to reduce power consumption while the electronic apparatuses are not being operated at a standby state, that is, in a operation standby mode in consideration of environment, for example. Here, for example, when the X-capacitor 40 has a capacitance of 1.0 μF, the requested value of the discharge resistance 20 of the discharging circuit is equal to or lower than 1 MΩ. For example, when commercial power supply voltage is AC 230 V, the power consumption by the discharge resistance is about 52.9 mW. The power consumption is not ignorable in an operation standby mode. A discharging circuit or the zero-crossing detecting circuit having the X-capacitor 40 and the discharge resistance 20 consumes power at all times because current is fed to the discharge resistance of the discharging circuit independent of the driving state of the electronic apparatus.
For example, according to Japanese Patent Laid-Open No. 2005-201587, a zero-crossing detecting circuit is turned on and off in an operation standby mode to reduce the proportion of the ON period. If the zero-crossing detecting circuit detects removal of the AC plug, the proportion of the ON period is increased more than the operation standby mode (or keeps the ON state). This may reduce the time for discharging residual electric charges by the X-capacitor 40 and may reduce the power consumption by the zero-crossing detecting circuit in the operation standby mode.
According to a different circuit configuration from Japanese Patent Laid-Open No. 2005-201587, a standby-only power supply 11 may be provided separately from the main power supply 12, for example, as illustrated in FIG. 17B. In an operation standby mode, a line from a commercial power supply of the main power supply 12 may be turned off to disconnect an X capacitor 44. In FIG. 17B, discharge resistances 20 and 30 and X capacitors 40 and 44 are provided. In a normal operation mode, power is supplied from the main power supply 12. In the operation standby mode, the main power supply 12 is turned off by a switch 73, and power is supplied from the standby-only power supply 11. In the operation standby mode, the electronic apparatus requiring a small amount of power may allow a lower capacitance of the X-capacitor 40 than that of the X-capacitor 44. In other words, the discharge resistance 20 may be larger, and the power consumption in the standby mode may be reduced.
As described above, circuits configured to discharge an X capacitor are devised for reduction of power consumption. However, because the configuration of Japanese Patent Laid-Open No. 2005-201587 uses a zero-crossing detecting circuit also as a discharging circuit, the zero-crossing detecting circuit may not be turned off in all periods. In other words, power is consumed which depends on the ON period of the zero-crossing detecting circuit. Further, because of the period when the zero-crossing detecting circuit is turned off, when external noise changes the AC voltage a commercial AC power supply, there may be a risk to mis-detect the timing of zero-crossing or there may be a risk that some timings may occur at which zero-crossing is not detectable.
A circuit configuration 10 may be considered which has a special power supply 11 for a standby mode as illustrated in FIG. 8. However, even in a standby mode, an X-capacitor may often be required. Assuming the capacitance of the X-capacitor is equal to 1.0 μF, the required discharge resistance is 1 MΩ. In this case, the power consumption by the discharge resistance is about 52.9 mW when the AC voltage of the commercial AC power supply is AC 230 V. When the capacitance of an X-capacitor in the circuit configuration 10 having the special power supply 11 for a standby may be reduced to 0.22 μF, the required discharge resistance is equal to or lower than about 4.5 MΩ. The power consumption by the discharge resistance is about 11.8 mW when the AC voltage of the commercial AC power supply is AC 230 V. The configuration has limitations to further reduce the power consumption during a standby mode though the power consumption may be reduced more than configurations in the past. The circuit configuration having the special power supply 11 for a standby mode naturally has more circuit components of the special power supply, increasing the circuit costs.
The present invention was made in view of the problem and may reduce power consumption in a standby mode with an inexpensive configuration and allows quick discharge of residual electric charges when power supply from a commercial power supply shuts down because of removal of a plug, for example.